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Acid-Base Analysis W. Rose See Marieb & Hoehn 9 th ed., Chapter 26

Acid-Base Analysis W. Rose See Marieb & Hoehn 9 th ed., Chapter 26 . Normal Arterial Blood Gas pH = 7.35-7.45 P a CO 2 = 35-45 mmHg HCO 3 - = 22-26 mEq /L P a O 2 = 80-100 mmHg We will skip base excess and anion gap. Abnormal Blood Gases Arterial pH<7.35: Acidosis

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Acid-Base Analysis W. Rose See Marieb & Hoehn 9 th ed., Chapter 26

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  1. Acid-Base Analysis W. Rose See Marieb & Hoehn 9th ed., Chapter 26

  2. Normal Arterial Blood Gas pH = 7.35-7.45 PaCO2 = 35-45 mmHg HCO3- = 22-26 mEq/L PaO2 = 80-100 mmHg We will skip base excess and anion gap

  3. Abnormal Blood Gases Arterial pH<7.35: Acidosis Arterial pH >7.45: Alkalosis

  4. Arterial pH < 7.35: Acidosis Is it respiratory, metabolic, or both? If PaCO2 abnormally high (>45 mmHg), acidosis has a respiratory cause. PaO2 likely to be low (<80 mmHg). If HCO3abnormally low (>22 mEq/L), acidosis has a metaboliccause.

  5. Arterial pH > 7.45: Alkalosis Is it respiratory, metabolic, or both? If PaCO2 abnormally low (<35 mmHg), alkalosis has a respiratory cause (hyperventilation). PaO2 likely to be high normal (95-100 mmHg). If HCO3abnormally high (>26 mEq/L), alkalosis has a metabolic cause (gain of strong acid, or loss of base).

  6. Arterial blood gas diagram

  7. Compensation There is immediate but limited compensation from the bicarbonate buffer system of blood. Lungs can compensate (in minutes) for a metabolic disturbance. Increased breathing to compensate for metabolic acidosis is a more robust and reliable response than decreased breathing in response to metabolic alkalosis. Kidneys can compensate (in hours to days) for a respiratory disturbance. Renal compensation for respiratory acidosis is slow but may be nearly complete after 3-4 days; renal compensation for respiratory alkalosis is slow and incomplete.

  8. Compensation Abnormally low PaCO2 (<35 mmHg) in metabolic acidosis indicates respiratory compensation (hyperventilation). Pure (uncompensated) metabolic acidosis seldom seen since respiratory system compensates quickly. PaO2is often high normal (95-100 mmHg) when there is respiratory compensation. Compensated (or chronic) metabolic acidosis pH=7.25, PaCO2=25 mmHg, HCO3=10 mEq/L There’s respiratory compensation It’s acidosis The acidosis is metabolic

  9. Compensation Abnormally high PaCO2 (>45 mmHg) in metabolic alkalosis indicates respiratory compensation (hypoventilation). PaO2 may be normal or slightly below normal (<=80 mmHg) Uncompensated metabolic alkalosis pH=7.58, PaCO2=44 mmHg, HCO3=40 mEq/L Compensated metabolic alkalosis pH=7.50, PaCO2=55 mmHg, HCO3=40 mEq/L There’s respiratory compensation It’s alkalosis The alkalosis is metabolic

  10. Compensation Abnormally high HCO3 (>26 mEq/L) in respiratory acidosis indicates renal compensation. Uncompensated (or acute) respiratory acidosis pH=7.25, PaCO2=60 mmHg, HCO3=25 mEq/L Compensated (or chronic) respiratory acidosis pH=7.35, PaCO2=60 mmHg, HCO3=32 mEq/L Abnormally high – this person isn’t getting rid of CO2 Near normal Higher than normal – kidneys must be compensating for the high CO2

  11. Compensation Abnormally low HCO3(<22 mEq/L) in respiratory alkalosis indicates renal compensation. Compensated (or chronic) respiratory alkalosis pH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L, PaO2=53 mmHg, pneumonia It’s alkalosis The alkalosis is not metabolic – quite the opposite The alkalosis is respiratory

  12. Sources 'Acid-base pHysiology' , K. Brandis, http://www.anaesthesiaMCQ.com GlobalRPh: Arterial blood gases. http://www.globalrph.com/abg_analysis.htm

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